1. Field of the Invention
The present invention relates to a method of processing a color image and an apparatus therefor.
2. Related Background Art
In a conventional color image forming system, primary color signals, e.g., Y, M, and C signals obtained by reading a color image are processed, the processed signals are subjected to UCR (Under Color Removal) to obtain Y, M, and C signals for printing colors on a recording medium and the Bk (black) signal obtained by multiplying a minimum value of the Y, M, and C signals with a predetermined ratio, and a color image is formed as a visible image on a recording medium such as paper by using Y, M, C, and Bk inks.
However, when a UCR amount or an inking amount is increased, a saturation level of a portion having a low density, i.e., a bright portion is degraded, and image quality is degraded. For this reason, a balancing point among a maximum saturation level of a low density, a "solid" black area in a high-density portion, and ink offset (transfer of an ink to an object except for the recording medium) must be found to determine a UCR amount and an inking amount.
In order to cope with these problems, a journal of the Institute of Electrophotography, vol. 24, PP. 60-67 describes a technique wherein a threshold value is set for a UCR amount, and UCR and inking operations are not performed until a minimum value K.sub.0 of the Y, M, and C signals exceeds a threshold value T. UCR and inking operations are performed only when the minimum value K.sub.0 exceeds the threshold value T.
In processing using such a threshold value T, in order to obtain a maximum black ink amount in a high-density portion and eliminate offset, a gamma correction level of a black signal must be set at a higher level (dotted line (2) in FIG. 4) as compared with a case wherein the threshold value T is not given as indicated by a solid line (1) in FIG. 4. A black ink amount is plotted along the ordinate of FIG. 4, while the minimum value K.sub.0 is plotted along the L abscissa When the minimum value K.sub.0 =min(C,M,Y) exceeds the threshold value T, an amount of black ink is abruptly increased, and printing is started. Black has a higher contrast level than those of cyan, magenta, and yellow when these colors are visually observed. A start black portion upon printing tends to be typically noticed as compared with the remaining black portion, as if a pseudo edge is present between the start black portion and the remaining black portion. For this reason, the UCR amount and the inking amount cannot be sufficiently increased in this method.